RT_GROUP_SCHED kernel option makes JACK unusable in Ubuntu Natty

Ronan Jouchet ronan at jouchet.fr
Tue Dec 14 04:34:21 GMT 2010

Hi Ubuntustudio-devel, Ubuntu-kernel, JACK-devel, Alessio,

After talking with adi in #ffado and las in #jack, it appears that 
Natty's kernel needs adjustment to be usable for audio work. Currently 
the kernel flavors shipped by Ubuntu have the RT_GROUP_SCHED option 
activated, which makes JACK unusable (las put up 
http://jackaudio.org/linux_group_sched to sum up the situation).

*Until this problem is solved, JACK is unusable under Natty*. Las 
mentioned several solutions:
   0. Disabling RT_GROUP_SCHED for all kernels
   1. Disabling RT_GROUP_SCHED for the kernel targeting audio users 
(-lowlatency, currently living in abogani's PPA)
   2. Set up a cgroup and invoke JACK and every client from a shell 
script that joins the cgroup first
   3. Use cgrules.conf to at least put processes into the right group by 
   4. Use Ubuntu's specific mechanism to put user tasks into a 
RT-enabled cgroup

Quoting las: "(1) is the desirable option; (2) would be next and (3) 
would be hard, because it would enforce 'application <foobar> always run 
in the <bazbar> cgroup' which is not really very useful or correct". (0) 
may also be considered since this kernel option is apparently very 
specialized for embedded systems and doesn't make sense for a desktop 

Which path should be followed is beyond my knowledge. I just hope this 
discussion will pave the way to a usable audio stack in Natty.

   LP bug: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/690010
   IRC : #jack, #ubuntustudio,  #ubuntustudio-devel
   IRC log of my discussion with las: http://pastebin.com/2yw7aPhr

All the best,

Ronan Jouchet

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